irqchip/gic-v3-its: Limit scope of VPE mapping to be per ITS

So far, we map all VPEs on all ITSs. While this is not wrong,
this is quite a big hammer, as moving a VPE around requires
all ITSs to be synchronized. Needles to say, this is an
expensive proposition.

Instead, let's switch to a mode where we issue VMAPP commands
only on ITSs that are actually involved in reporting interrupts
to the given VM.

For that purpose, we refcount the number of interrupts are are
mapped for this VM on each ITS, performing the map/unmap
operations as required. It then allows us to use this refcount
to only issue VMOVP to the ITSs that need to know about this
VM.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>